The Discovery of Halley-sized Kuiper Belt Objects Using HST

We report the statistical detection (at the > 99% confidence level) of a population of 28th magnitude objects exhibiting proper motions of ≈1 arcsecond per hour at quadrature in deep HST/WFPC2 images. The drift directions imply a preponderance of objects on prograde orbits concentrated near the ecliptic plane. We interpret this as the detection of objects which reside in the Kuiper belt near or beyond the orbit of Neptune, comparable in size to comet 1P/Halley (radii ∼ 10km for an albedo of 4%). Our observations imply a population of ∼ 2.5× 10 objects deg with V < 28.6. This must be viewed as a lower limit until our search of parameter space is complete. Our observations imply that there are > 2× 10 objects of this size in the Kuiper belt with inclinations < 12◦ and within ∼ 40au of the Sun. Subject headings: comets: general – solar system: formation – solar system: general